The present invention relates to a rubber composition for cap tread and a tire having cap tread comprising the same.
The low fuel cost of a car has been carried out by reducing the rolling resistance of a tire and suppressing heat build-up, in recent years. In particular, request for the low fuel cost of a car has been strengthened and more superior low heat build-up property is requested for tread having high occupied ratio in a tire among tire members.
Although superior grip performance is requested for the tread of a tire from the viewpoint of safety in the running of a car, the low fuel cost and grip performance are contradictory performance; therefore in general, both of the low fuel cost of a tire and grip performance are satisfied by making tread as the two layers structure of cap tread and base tread, adopting rubber having low heat build-up property as the base tread, using a rubber composition having high grip force for surface layer portion that is brought in contact with road surface such as the cap tread and using a rubber composition having low heat build-up property for inner layer portion such as the base tread.
As a method of reducing the rolling resistance of tire tread, there are mentioned a method of compounding the specific amount of two kinds of silica having different nitrogen adsorption specific surface areas (for example, Patent literature 1) and a method of compounding a modified styrene-butadiene rubber as a rubber component (for example, Patent literature 2).
Although it is also known as a rubber composition for cap tread that heat build-up property is suppressed by increasing the compounding amount of sulfur, the fracture strength TB and elongation at break (EB) are lowered when the compounding amount of sulfur is increased; therefore there have been problems that tread crack is generated and the life time of tire tread is shortened when the resulting rubber composition is used for the cap tread for tire.
Namely, fracture strength and low heat build-up property are contradictory physical property and it was difficult to satisfy both physical properties.    [Patent literature 1] Japanese Unexamined Patent Publication No. 2006-233177    [Patent literature 2] Japanese Unexamined Patent Publication No. 2006-56979